1. Field of the Invention
This invention relates to tamper proof critical temperature verification apparatus and more particularly to critical temperature verification apparatus having a permanently sealed casing within which a temperature indicator can be permanently locked in visible position by cooperation of a locking spring and a cam and a shoulder adapted to engage the locking spring at the critical temperature.
2. Description of the Prior Art
In the storage, transportation and use of many materials such as food, medical vaccines, bonding agents, insulating materials, industrial compounds and mixtures, and the like, it is important to know when, or if, a preselected or critical high or low temperature condition has occurred. Various temperature indicators have been suggested to record occurrence of critical temperature conditions. U.S. Pat. No. 4,081,999 describes such a device having a temperature responsive bimetallic coil connected to a rotating indicator which traverses a temperature scale having selected portions of the face of the scale bent outwardly to define an inclined plane member in the path of the indicator. The indicator is sufficiently resilient to permit unobstructed movement over the inclined plane in one direction while insuring latching behind the outwardly bent inclined plane upon movement in the opposite direction. That device permits easy and arbitrary manual resetting of the indicator and is so constructed that movement or vibration could dislodge the indicator and destroy the critical temperature verification.
U.S. Pat. No. 3,291,617 discloses a device having a spring loaded, temperature responsive bimetallic indicator strip enclosed in a circular case having upper and lower case members. The strip is held in original position by a first stop member extending inwardly from the upper case member until the attainment of a preselected minimum temperature which causes bending of the strip to permit its passage past the first stop member. The strip is then held in position by a second stop member extending inwardly from the lower case member until increasing temperature bends the strip in the opposite direction thus permitting passage of the strip past the second stop member into a final resting warning position. That device can be improperly activated by movement, vibration, or physical shock and can be readily and arbitrarily reset.
In an advertising brochure published in 1977 by William Wahl Corporation of Los Angeles, Calif., No. W-610D, Revision B, entitled "Wahl Surface Temperature Thermometers", there are described several temperature devices designed for mounting on the surfaces of industrial machinery and equipment. Certain of the devices described therein have a temperature responsive bimetallic coil connected to a rotatable pointer which traverses a temperature scale and which is constructed to drive, in a single preselected direction, a color coded recording hand which records either a maximum or minimum temperature reached since the last setting. That device permits easy, arbitrary manual resetting of the recording hand at the discretion of the user or operator.
U.S. Pat. No. 2,966,261 describes a spring loaded indicator temporarily maintained in position by a temperature responsive bimetallic keeper element which flexes with temperature change to release the spring loaded signal element. The device is enclosed in a sealed housing which inhibits tampering and precludes resetting of the device. A retaining rod, inserted through the device during shipment, prohibits normal action of the device until the retaining rod is removed. The free end of the spring loaded signal element engages a fixed keeper element until temperature variation causes the bimetallic signal element to disengage itself from the keeper and assume the warning position. With the retaining rod removed, vibration or physical shock can dislodge the spring loaded signal element from engagement with the keeper prematurely. In addition, the opening in the casing which previously held the retaining rod for shipment, must be sealed before use of the device to prevent tampering by reinsertion of the retaining rod or its equivalent to prevent movement of the signal element in response to temperature change.
U.S. Pat. No. 3,214,278 describes a device constructed to indicate attainment of a critical low temperature and subsequent attainment of a critical high temperature having a temperature responsive bimetallic element which, upon temperature decline, exposes a color coded low temperature indicator to view beneath a transparent window in the housing of the device. At the critical low temperature position, further movement of indicator elements in response to the decreasing temperature is prevented by a traverse member having a series of inclined planes. Subsequent increase in temperature causes the bimetallic element to drive the indicator elements in the opposite direction along the inclined planes until a second, high temperature indicator is moved into view beneath the transparent window in the housing. Subsequent temperature decrease will not cause removal of the high temperature indicator from view because the inclined plane prevents such reverse movement. Movement, vibration or physical shock can dislodge or otherwise disturb the structural elements of that device, thereby rendering inaccurate the temperature indication condition.
U.S. Pat. No. 3,813,942 discloses a washing machine having a high temperature indicator comprising a pointer attached to a bimetallic coil. Movement of the coil and pointer in response to temperature change is prevented by a stop rod that rests against a pivoted plate until the washing machine pump is activated to drain the machine. Activation of the pump displaces the pivot plate to release its frictional engagement with the end of the stop rod, thereby permitting movement of the coil and pointer to indicate the attainment of high temperature. Vibration or physical shock imparted to the washing machine during operation can cause malfunction of the temperature device. In addition, the locking means and temperature indicator are released upon activation of the next operational cycle of the washing machine and inadvertent activation of that cycle can cause unintentional destruction of the temperature verification.
There remains a need for a simply constructed, stable, tamper proof, critical temperature verification apparatus that provides a permanently locked and stable critical temperature indicator that is resistive to vibration or physical shock.